Chemical exposure at work.
Last month's Tech Service column (in the June 1990 issue of Rubber World) featured the first of a two-part article on chemical exposure in the workplace. Part 1 covered control of worker exposure to chemicals through a systems approach. Engineering changes, including material modification, automation/isolation, local exhaust ventilation and work practices, were discussed.
Part 2 of this column, presented here, will offer solutions to reduce worker exposure to chemicals in specific work areas, including compounding/weigh-up, mixing, secondary processing, fabrication/preparation, curing and final inspection/repair.
What about specific areas?
Control of chemical exposure, namely dusts, in this area is difficult even with automated, enclosed equipment. Aside from alterations in the materials as previously noted, one of the best ways to control exposure is through organization of the work area and effective LEV.
The work area should be organized so that powders do not have to be carried far between the bag or drum and the scale. Weighing into the container to be used for transport rather than weighing into a pan and dumping would be preferred. Plastic bags that can be sealed after loading and opened immediately prior to adding to the mix can work well.
Ventilation must be arranged so that air flow is maximized in the area of greatest need. Slot vents directly adjacent to the weigh container can be reasonably effective. Canopy hoods may also be used. One of the primary problems with most LEV systems in compounding areas is that the ventilation is located at the scale and rarely does any ventilation occur around the container from which the powder is being removed and excess returned. As a result, the dust generated in removing and returning powders to and from the storage container exceed that generated in the actual weighing operation. As a result, some have suggested that the only real solution in this area is automation. I would suggest that through proper use of air flow and possibly different type of ventilation, this would not be the case.
Typically, mixing involves the mixing of powders, oil and rubber in high shear internal mixers. However, in many operations, mills are still utilized for mixing rubber compounds.
Internal mixers are much easier to deal with. There is less open area for the dusts and fumes to be generated. Large plants can utilize bulk handling systems for many of the powders and oils. However, the smaller operations are still limited economically to handling bag fillers (black, clay, etc.) and drum oils.
When possible in the formulations, utilize fillers that are pre-packaged into low melt polyethylene bags. Bag and all can be dropped into the mix. The same can be done with other chemicals and some oils that are preweighed for addition.
The American Conference of Governmental Industrial Hygienists recommends that the slot type hoods over the throat be replaced with canopy type hoods that cover the entire loading area (figure 1). This is the major problem area.
Local exhaust may also be used around the drop/sliding gate and the dust rings. Proper maintenance of these items can be very important in controlling atmospheric dust.
Mill mixing provides significant challenges. Control of both dust and spills can be accomplished to some degree by use of oiled or predispersed fillers and by selecting a mixing method that allows addition of oils with the dry, dusty materials.
Aside from this, local exhaust is the best method (figure 2). Canopy type hoods are the most common for mixing mills. Additional containment, however, can be achieved by enclosing the open area around the sides of the hood and behind the mill. This can be done using plastic sheets, sheet metal or even heavy cloth drapes. The latter are not recommended since they tend to collect materials and can lead to additional exposure problems as well as contamination problems.
All of these measures are well within the realm of capability of even the smallest manufacturers.
These operations involve the re-milling, extrusion and calendering of materials for subsequent building and curing operations. In the case of continuous vulcanization lines, secondary processing and curing take place at one time.
One of the problems in secondary operations involves fumes generated off the rubber as it is heated and formed to shapes. Chemical content of these fumes varies. Whether their content is harmful or not is open to question. However, prudence along with the observance of Jon's second law would suggest that the fumes should be avoided. About the only alternative available here is ventilation.
Hot fumes rise. Canopy hoods will work best, keeping them as close to the work area and machinery as possible. For milling, canopy hoods with enclosed sides (and back) will function well. For calenders, ventilation is needed on each side with positive make-up air flows coming from behind the worker.
For extrusion lines, a variety of ducting systems can be considered. Exact configuration will depend on what is being run. In some cases, canopy hoods will be the most effective. However, if the material being run requires the operator to bend down closely over the work for quality checks, etc., a side draft type duct with relatively high air flow may be best.
The second problem in this area is dust, primarily from the anti-tack agents used. Many factories use kaolin type clay coatings. Still others may still be using various forms of zinc stearate or even dry talc. All of these can cause dust problems as the material is processed.
Since dust generation from coated rubber slabs will occur wherever the material is handled and since this can happen over a wide range of floor space, local exhaust would not be expected to be a very effective solution to the problem. Replacement of the dry anti-tack agents with soap may be acceptable and will significantly reduce dust generation in the area. Acceptability of this change will vary depending on the material being processed.
Other alternatives include changing the formulation of the anti-tack agent (higher film strength) or use of polyethylene sheeting. Typically, the latter of these is an expensive alternative and increases the disposal problem. The first of these generally works well with higher hardness materials but can be a problem (dispersion of the coating) with lower hardness materials.
The fabrication/preparation step actually can involve a number of processes. Normally, the rubber is in calendered sheet, milled or extruded form with little or no anti-tack agents on the surface. In some cases, warming tables may be used to raise the temperature somewhat over room temperature, but as a practical matter, the temperature is never high enough to cause generation of fumes. Assuming that proper gloves and skin protection are provided, there is little danger of chemical exposure from the rubber.
The major problem in this area is with solvents. These, of course, are used to wash the rubber surface or as diluents for adhesives. In large plants, such as tire plants, it is possible to automate most cement operations. This includes mixing of cements as well as application to the rubber, such as a tread.
In the smaller operations, automation is not normally feasible. The best engineering means here include: 1) containment (keep containers closed), and 2) ventilation, with 3) proper protective clothing, and 4) respirators as back-ups. Keeping containers closed, making sure lids are kept on mixers, making sure valves and spigots don't leak are the primary means available for containment. Ventilation, including providing proper clean make-up air in the areas of use, is very important. Work areas should be designed so containment hoods can be placed over and around each of the mixing units.
In production areas, ventilation considerations will vary significantly. For operations that are applying cements by spray techniques, commercial spray booths are available that are very effective at reducing solvent exposure. In areas where cements are being applied by hand, considerable thought needs to be given to the layout of the work area. Use of canopy hoods, side draft ventilators and creative use of baffles to create artificial paths for air flow will reduce exposure.
A positive clean air source needs to be provided behind the work area to create the proper flow. In this area, having a clean air source must be emphasized. Aside from keeping workers healthy, any dust or other contamination present in the air stream can contaminate the surface of the rubber and cause manufacturing problems. Depending on the layout of the shop, it may be necessary to duct fresh air into the area.
In all the areas where cements and solvents are handled, there needs to be overall general ventilation to control exposure to minor spills and leaks that will occur.
There are a number of sources of chemical exposure in this area. Lubes applied to parts, molds, etc., and curing fumes. Proper design of the air flow patterns for the work area along with controlled work practices are probably adequate for control of lubes. However, control of curing fumes takes more consideration. One of the unfortunate and bothersome facts about curing fumes is that their make-up will depend on the formulation of the rubber as well as cure temperatures being used.
There are a wide variety of ways curing is accomplished - molding, autoclave, open steam, continuous vulcanization, etc. Continuous vulcanization lines are generally supplied with excellent hood ventilation systems to remove fumes generated during the curing process. They tend to present little problem.
Gas type autoclaves, where the part being cured is vacuum bagged and an inert gas such as carbon dioxide or nitrogen is used as the pressure/heating medium also present little problem. Fumes generated in curing are removed by the vacuum system.
Live steam autoclaves, on the other hand, where the rubber is exposed to the steam or wrapped only with a cloth, allow any fumes generated in the cure to mix with the steam in the autoclave. While blow down of the clave following cure will remove most of these, there will still be a volume of steam and curing fumes released into the work area when the autoclave is opened. Control of these requires the following:
1) Venting blow down lines outside and away from work areas.
2) Using high volume ventilation over the door area of the autoclave.
3) Providing a fresh air source for the autoclave.
4) Allowing sufficient time for fumes to be removed before carts, trolleys, etc., are removed from the autoclave.
Molding operations also provide a significant source for worker exposure to curing fumes. Most tire curing anymore is almost completely automated, allowing the operator to stay some distance from the press when it opens. However, most industrial molding operations require workers to be standing close to the presses when they open.
The best approach to avoid exposure is through design of the work area/ventilation system and proper training of employees. By designing the work area so that exhaust ventilators are behind the press, baffles are placed over or just in front of the presses and a fresh air source is provided behind the worker, exposure to curing fumes can be minimized. For horizontal type injection molding machines, canopy hoods over the mold opening area may be sufficient.
This area will tend to combine the problems mentioned in many of the previous areas but to a more limited degree. Buffing, rubber dusts, repairs involving uncured rubber stocks, cements, curing fumes, etc., may all be present. Layout and design of the work area with an eye to air flow patterns is one of the most effective means of control. In most of these operations, automation (with the exception of trimming) is difficult if not impossible, requiring management to rely on ventilation and personal equipment.
So what does it all mean? You say, "I've been in business 15 years and haven't had a problem," or "We're too small. Nobody will bother us." Well, maybe.
In our business, we deal with a very great number of chemicals. These include nuisance dusts, to some materials that are quite hazardous. On March 15 of this year, OSHA published a Health Hazard Bulletin on N-Nitroso compounds, largely focused on our industry. These compounds are often generated in the manufacturing process, as reaction products from amines used in curing, antioxidants and antiozonants and other chemicals. In a sampling of rubber manufacturing operations done by NIOSH, contamination was found in virtually all factory areas.
In a news release on May 1, 1990, Secretary of Labor Elizabeth Dole announced her support for legislation to triple the maximum limits for OSHA civil penalties and upgrade criminal violations.
We live in a highly litigious society. In California, where many of us live, I suspect there are more attorneys than real estate agents. And attorneys, like accountants, are a self perpetuating species. No matter how big or small we are, we need to be aware of the problems involved in exposing our workers and the public in general to the chemicals we use.
We are an old industry and often rely on what was good enough 10 or 20 years ago. Unfortunately, that is not good enough today.
Big or small, action is required to improve the health and working conditions in many if not all of our factories. And it's much better to do it before that attorney or OSHA inspector knocks on our door. [Figures 1 to 2 Omitted]
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|Title Annotation:||part 2|
|Date:||Jul 1, 1990|
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